Being a fictional short story, there isn't an actual thesis in the work. If I were going to choose one sentence that emphasizes the meaning of the story it would be the very last sentence.
When the doctors came they said she had died of heart disease--of joy that kills.
This sentence points to the irony in the story. We as readers know that Mrs. Mallard didn't die joyfully, but rather at the shock and disappointment that her husband was still alive.
If you are looking for a thesis, as in the main point, of the story, it would be that women are oppressed in marriage relationships. In most of her works, Chopin was concerned about showing the role of women in society and in marriages.
Although Brently Mallard was not physically abusive, he was very controlling